Functional potentiometer



June 18, 1957 ENGELDER 7 2,796,502

FUNCTIONAL POTENTIOUETER Filed 061.- 26, 1953 3 Sheets-Sheet 1 4072/02 A iNL NGZLDZE June 18, 1957 A. E. ENGELDER 2,796,502

FUNCTIONAL POTENTIOHETER Filed Oct. 26, 1953 3 Sheets-Sheet 2 pen/04 A. [A/GELDE? INVENTOR.

A77 zueu June 18, 1957 ENGELDER 2,796,502

FUNCTIONAL PQTENTIOMETER Filed 00?.- 26, 1953 3 Sheets-Sheet 3 raw/5:71am p- 9/ 45 y 92 4272/02 5. Euaaoi INVENTOR.

United States Patent FUNCTIONAL POTENTIOMETER Arthur E. Engelder, Douglas, Aria.

Application October 26, 1953, Serial No. 388,388

8 Claims. (Cl. 201-60) This invention relates to a function generator, and particularly to a variable resistor for use as a potentiometer or rheostat in producing non-linear functions.

In recent years it has become customary to construct a variable resistor by providing a hel'ically wound resistance element on the cylindrical interior wall of a surrounding casing or mandrel. The contact element or brush for such a structure is mounted on a shaft journaled axially of the helica'lly wound resistance element, so that the electrical contact is between the outer portion of the brush and the inner surface of the resistor. Although such a structure has proved relatively satis factory for generating linear functions, substantial manufacturing and operating difficulties are encountered when it is attempted to construct such a variable resistor so that it will generate a non-linear function, particularly when the function is one involving steep curvatures and numerous points of inflection. This is because it is extremely difiicult and expensive to 'form a mandrel the interior wall of which is shaped in accordance with a complex non-linear function, and also because it is impractical, with such an arrangement, to construct a brush which will adequately compensate for the varying distance "between the centrally disposed shaft and the adjacent portion of the encircling resistor.

With respect to convention-a1 function generators or variable resistors employed as rheostats as distinguished from potentiometers, the resistance element and the associated brush are constructed so that the turning or shifting of the movable element in a given direction will result in either a progressively increasing or decreasing resistance value. Thus, if it is desired to generate a function in which the values alternately increase and decrease, it is necessary to reverse the direction of relative movement at each point of inflection. It is additionally pointed out that with a conventional rheostat the shifting of the movable component in a given direction will always result in either an increase or a decrease in the resistance in the circuit, it being impossible, without stopping the relative movement, to' generate a function having .an unvarying or constant portion. Furthermore, in a conventional rheostat the resistance at any position is the sum of all the turns from the low end of the winding to its point of contact with the brush, which makes it necessary to reverse the direction of movement in order to subtract resistance from the circuit. The subtraction of resistance is from a fixed maximum, and any adding of resistance to the circuit is always from a fixed minimum, instead of from maximum and minimum points which may be varied.

'In view of the above factors characteristic of function generators of the variable resistance type, it is an object of the present invention to provide a variable resistor which may be economically manufactured with extreme precision and in such a Way as to generate any one of innumerable non-linear functions.

An additional object is to provide a rheostat capable of generating both increasing and decreasing values with- A 2,796,502 Patented June 18, 1957 out reversal of the direction of movement of the movable rheostat component, which may be employed to generate a constant value during movement of the movable component, and which is such that the upper and lower limits of the rheostat may be continuously varied as may be desired.

Another object of the present invention is to provide a variable resistor adapted to generate non-linear functions through cooperation between an exterior surface of revolution and pick-up means adapted to move radially and longitudinally of the surface of revolution.

An additional object is to provide a device having a mandrel formed with an exterior surface of revolution, the mandrel being rotatable to effect variation in the resistance or voltage picked off by a non-rotatable contact element associated with a resistance element on the exterior mandrel surface.

Another object is to provide a variable resistor in which the resistance element is formed as an exterior surface of revolution, and in which the associated contact element is constructed with a base portion guided along a fixed path parallel to the axis of the surface of revolution and with a resilient contact portion adapted to move radially of the surface to maintain contact with high or low points thereon.

An additional object is to provide a rheostat the resistance portion of which is formed on a non-cylindrical surface of revolution, and the contact portion of which is constructed with two mutually insulated elements adapted to engage the resistance portion at spaced points thereon, so that relative movement between the contact portion and the resistance portion will effect variation in the amount of resistance between the two insulated elements in accordance with the curvature of the surface of revolution.

A further object is to provide a variable resistor having a resistance element wound on a rotatable mandrel formed as a non-linear, non-cylindrical surface of revol-ution, and having a brush portion driven axially of the mandrel and additionally adapted to shift both radially and axially thereof to compensate for variations in mandrel diameter and in the location of the resistance element thereon.

These and other objects and advantages of the invention will be more fully set forth in the following specification and claims considered in connection with the attached drawings to which they relate.

'In the drawings:

Figure 1 is a longitudinal central sectional view of an embodiment of the present invention;

Figure 2 is a view, partly in section and partly in end elevation, as viewed from the broken line 2'2 of Figure 1 and in the direction of the arrows;

Figure 3 is a transverse sectional view of the mandrel and taken at line 3-3 of Figure 1, showing portions of the static contact elements for the tap leads;

Figure 4 is a transverse sectional view of the mandrel and the brush assembly and taken at line 4-4 of Figure 1, illustrating the resilient contact arms in several radial positions;

Figure 5 is an enlarged longitudinal sectional view taken at line 55 of Figure 4;

Figure 6 is an enlarged detail view taken at line 6-6 of Figure 4, showing the 'brush end portion for engaging both the mandrel and the resistance element wound thereon;

Figure 7 is an enlarged isometric view of one of the resilient contact arms of the brush structure shown in' of a brush arm, looking outwardly from the contacted surface of the mandrel;

Figure 9 is a transverse sectional view of the mandrel, taken along line 99 of Figure 1 and showing the static connection components adapted to engage the components illustrated in Figure 3;

Figure .10 is 'an isometric view of a static connection component illustrated in Figure 9;

Figure 11 is aschematic illustration showing the connection of the electrical components of the invention as a potentiometer;

Figure 12 corresponds to Figure 11 but shows the electrical components connected as a rheostat; V

Figure 13 is a fragmentary longitudinal sectional view of a modified form of mandrel and ring-contacting brush; and

Figure 14 is a view corresponding to Figure 6 but illustrating a modified form of mandrel and brush structure in which the resistance element is wound in one of a pair of independent grooves and the mandrel-engaging components of the resilient brush are adapted to engage the shoulders of the other groove.

Stated generally, and referring to the drawings, the

variable resistor comprises a mandrel formed as a surface of revolution about a central axis along which is disposed a shaft 21, the latter being operable when rotated by a suitable manual or servo arrangement, not shown, to effect rotation of the mandrel. Although the mandrel may be of a solid volume conductor type, a graphite film type, or an auto-transformer type, it is preferably of a wire-wound type having a resistance element 22 wound as a helix on its exterior surface. To eifect electrical contact with the various portions of resistance element 22, the pick-up or brush assembly 23 is provided and comprises a base 24 adapted to move along a path parallel to the mandrel axis and in synchromism with mandrel rotation, and resilient arms 26 and 27 extending from the base 24 transversely of the mandrel and into engagement with the resistance element. It is a feature of the invention that the arms 26 and 27 not only move longitudinally of mandrel 20 with base 24 as the latter is driven, but also may move radially of the mandrel and may be both flexed and pivoted longitudinally thereof to insure proper contact between the arms and resistance element. It is an additional and very important feature of the invention that arms 26 and 27 may be. connected in an electric circuit as a rheostat, with the resistance inserted between them either increasing or decreasing depending upon whether the mandrel diameter at the points of contact is becoming larger or smaller.

Proceeding now with a more detailed description of the above and other important elements of the invention, and with particular reference to Figure 1, the apparatus is shown 'as mounted in a rectangular casing having a base wall 29, a removable cover plate 30, and rigidly associated end walls 31 and 32. Walls 31 and 32 are parallel to each other and perpendicular to shaft 21, the latter being journaled therein by means of suitable ball and thrust bearing assemblies 33. The mandrel 20, which may be cylindrical but is ordinarily any one of innumerable non-cylindrical surfaces of revolution about shaft 21, is illustrated in the present instance to comprise a wide end portion, to the left of Figure 1, which tapers to a narrow point of inflection indicated at 34. From the point of inflection 34, mandrel 20 progressively widens to a high point of inflection 36 and then narrows again to a third point of inflection adjacent the relatively narrow right end of the mandrel. The wide and narrow ends of mandrel 20 are respectively formed with end flanges 37 and 38 adapted to receive contact and spacer rings as will be described below. It is to be understood that the mandrel for the wire-wound resistor illustrated may be readily machined from metallic or nonmetallic stock, it being preferable to employ a plastic substance and one which is insulating to eliminate the necessity for providing separate insulation beneath resistance element 22.

To provide for the accurate mounting of mandrel 20 on its shaft 21, annular spacer rings 40 and 41 are seated in corresponding recesses in the wide and narrow ends of the mandrel, the spacer rings being press fit or otherwise mounted over the shaft 21 so that they will rotate with it. At the bottom of the recess for spacer ring 40 is formed an annular groove 42 adapted to receive a pair of static contact assemblies 43. The groove 42 is between outer and inner shoulders 44 and 45 against which the spacer ring 40 is seated. The inner shoulder or ridge 45 is formed with radial grooves 47 (Figures 3 and 9) to receive tap leads 48 and 49 which are respectively connected between the static contact assemblies 43 and the resistance element 22 at the points of inflection 34 and 36. To receive the tap leads, an annulus 51 is provided between shaft 21 and the mandrel 2t), and radial bores are formed between the annulus and the points of inflection. It is to be understood that the spacer rings 40 and 41 are either press fit into their associated recesses, or are suitably screwed or bolted to the main body of mandrel 20 to effect its rotation when shaft 21 is turned.

The resistance element 22 is shown in Figure 6 to comprise a cylindrical core 53 which is formed of a suitable flexible insulating material and has a relatively small resistance wire -54 wound helically thereon. Although the illustrated coiled-coil construction is preferred since it permits the insertion of a greater amount of resistance per unit length of the element, it is evident that an ordinary resistance wire may be substituted for the compound resistance element without departing from the invention. In order to receive the resistance element 22 and to effect guiding of the resilient contactor arms 26 and 27 as will be described subsequently, the exterior surface of mandrel 20 is formed with a relatively large helical groove 55 of semi-circular section, and with a relatively small groove 56 formed in the bottom of groove 55. As is shown clearly in Figure 6, the resistance element 22 is wound in the relatively small groove 56 in such a way that its upper half protrudes into large groove 55 for engagement by a portion of either of contact arms 26 or 27, there being other portions of the contact arms adapted to ride on shoulders 57 formed between the volutions of large groove 55.

The brush or contact assembly 23 is screw-fed along a shaft 59 which is generally a single thread worm having a lead corresponding to the lead of groove 55 in mandrel 20. The shaft 59 is disposed parallel to shaft 21 and is suitably journaled, as by ball and thrust bearings 60, in the end plates 31 and 32 of the casing. As is shown in Figure 5, shaft 59 extends through a longitudinal cylindrical bore 61 in brush base 24, and cooperates with a ball 62 and adjustment or pressure screw 63 to effect longitudinal movement of contact assembly 23. The adjustment screw is threaded downwardly through the upper portion of base 24, with its lower end adapted to press ball 62 into the worm groove 64 and with its upper end disposed at a channel 66. The channel is formed in the upper surface of base 24, parallel to shaft 59, and is adapted to receive a guide bar 67 mounted between casing end walls 31 and 32. With the described arrangement, rotation of shaft 59 effects movement of base 24 longitudinally of the mandrel while guide bar 67 effectively prevents the contact assembly from rotating and insures that it has but a single degree of freedom. It will be apparent that the operative connection between base 24 and threaded shaft 59 may be varied by turning adjustment screw 63 to alter the amount of pressure exerted by ball 62 against the shaft, this adjustment being effected after removal of guide bar 67 to permit access to theadjustrnent screw.

According to the invention, shaft 59 and shaft 21 are associated with each other in such a way that the resilient arms 26 and 27 of the pick-up or contact assembly will track continuously along resistance element 22 as the mandrel is rotated; To this end the lead or pitch of worm groove 64 is made the same as the lead or pitch of the mandrel groove 55, as previously indicated, and the shaft 59 is geared to shaft'21' with a one-to-one ratio. Preferably, the gear connection comprises corresponding spiral gears 69 and 70 respectively provided on shafts 59 and 21 at the right ends thereof as shown in Figure 1, and a'relatively large idler spiral gear 71 meshed between gears 69 and 70 and mounted on a ball and thrust bearing 72 in casing wall 32. In operation, the shaft 21 is turned manually or throughuse of a suitable servo-mechanism, which operates to rotate mandrel 20 and, because of the one-to-one ratio between gears 69 and 70, shaft '59 in synchronism with it. The leads of the mandrel and shaft screw being the same, contact assembly 23 will then move longitudinally of the mandrel and in a direction to maintain its arms 26 and 27 in continuous engagement with resistance element 22.

Proceeding next with a detailed description of the very important means for associating contact assembly base 24 and the helically wound resistance element 22 and referring particularly to Figures 4, 6, 7 and 8, contact or brush arms 26 and 27 are formed of highly resilient electrically conductive material and are curved downwardly from the side walls of base 24, the latter being formed of insulating material to electrically isolate arm 26 from arm 27. Contact arms 26 and 27 preferably engage resistance element 22 on opposite sides of mandrel 20 and in a horizontal plane passing generally through shaft 21. Ordinarily, the arms 26 and 27 engage the resistance element at points separated by only a half-turn, so that a relatively short resistor section is bridged. However, for some applications of the variable resistor it may be desirable for arm 26 to engage the resistance element a turn and a half, or other suitable distance, from the point where arm 27 engages it. The arms 26 and 27 being identical and mounted symmetrically about a vertical plane passing through shafts 59 and 21, only the arm 26 will be described.

The curved body 74 of arm 26, which extends downwardly from a side wall of base 24 to the mid-portion of mandrel 20, is formed as a strip having any transverse axis parallel to shaft 21. The upper end of arm body 74 being substantially above the widest portion of the mandrel, it follows that the arm body may bend, for example from the solid to the dashed line positions shown in Figure 4, to compensate for variations in mandrel diameter. The strength and resilience of arm 26 is at all times sufficient to effect pressure engagement between its lower end and both mandrel 20 and resistance element 22 to effect tracking and electrical contacting functions as will next be described.

At its lower end, the arm body 74 is bifurcated to form outer guide portions 75 adapted to ride'on the outer cylindrical portions of mandrel shoulders 57. The guide portions 75 are bent inwardly to form lateral guide portions 76 which are curved to engage the side walls of groove 55 as best illustrated in Figure 6. Between the sets of guide portions 75 and 76, and spaced from them, is a U-shaped contact finger 77 formed integral with arm body 74 adjacent the bases of the guide portions. The contact finger curves first outwardly and then back between the ends of the guide portions to a position substantially inwardly of the guide portions where it terminates in a bent-over end 78. As previously indicated, the contactor arm 26 is designed so that its body 74 will have suflicient strength and resilience to effect pressing, at all times, of guide portions 75 and 76 against mandrel shoulders 57 and provide effective tracking as is desired. The contact finger 77, being substantially narrower than main body 74, has less strength and resilience and bends outwardly when its bent-over end 78 comes into contact with resistance element 22 just prior to engagement between the guide portions 75 and 76 and mandrel shoulders 57. With such an arrangement, the strength of contact finger 6 77" is solely determinative of the amount of pressure between its bent-over end 78 and resistance element 22.

If mandrel 20 were always a cylinder, the previously described construction would be completely adequate to maintain finger 77 in good electrical contact with resistance element 22 during rotation of the mandrel and consequent axial shifting of the contact assembly. However, the extreme non-linearity of many functions fre quently necessitates the use of mandrels having highly irregular surfaces of revolution, with relatively steep slopes. The axial movement of contact base 24 may not, under such conditions, be precisely the same as the lead of resistance element 22 as the mandrel is rotated. To insure effective electrical contact even in such situations, the body 74 of contact arm 26 is twisted at its upper end to form a short portion 79 lying in a plane perpendicular to shaft 21. The resilience of the arm 26 is then such as to permit not only radial but axial movement, the latter occurring upon flexing of short portion 79. Furthermore, short portion 79 is associated with base 24 by a screw connector 81 threaded into a correspondingly tapped hole 82 (Figure 4) in the base side wall. The fit of screw 81 into hole 82 is sufficiently loose to permit a slight turning of the screw and, consequently, a pivoting of contact arm 26 about an axis passing transversely through base 24 and perpendicular to shaft 21.

In electrically connecting the variable resistor into a circuit, only one of the arms 26 or 27 is ordinarily employed when the device is used as a potentiometer, although both arms may be electrically connected to each other and used as a single contact if desired. Both of the arms 26 and 27 may also be electrically connected to each other in order to adaptthe device for use as an ordinary rheostat. However, it is a very important feature of the invention that the arms 26 and 27 may be maintained electrically isolated and employed as a highly novel rheostat in which the spanned or bridged resistance both increases or decreases upon continuous turning of mandrel 20 in a single direction. It is to be understood that either or both of arms 26 or 27, as may be expedient, are suitably connected to a contact or terminal strip 83 (Figure l) to facilitate their connection into an electric circuit.

Referring again to Figure 1, the end connections for the resistance element 22 preferably comprise electrically conductive rings 84 and 85 which are suitably mounted, respectively, in annular edge-channels in mandrel flanges 37 and 38. The ring 84 is electrically connected to the resistance element end 86 at the wide portion of mandrel 20 by a screw 87 extending through flange 37 and into the ring, while ring 85 is similarly connected to the other resistance element end 88 by a screw 89 in flange 38 and the ring. The described construction provides smooth contact faces at the mandrel ends so that the rings 84 and 85 may be engaged, respectively, by suitable brushes 91 and 92 mounted on the casing end walls 31 and 32. As in the case of the contact arms 26 and 27, the brushes 91 and 92 are suitably connected through leads, not shown, to the terminal strip 83.

As previously indicated, the resistance winding 22 is tapped at the points of inflection 34 and 36, or at any other suitable points as may be desired, to provide either points of inflection in the output voltage or sharp breaks in the curve generated. Thus, for example, the section of resistance element 22 between contact ring 84 and tap lead 48 may be connected across a given voltage, the section between tap leads 48 and 49 across a second voltage, and the section between tap lead 49 and contact ring 85 across a third voltage. The end connections for tap leads 48 and 49 comprise contact rings 94 and 95 embedded in the outer face of spacer ring 40 at the wide end of the mandrel. Contact rings 94 and 95 are respectively engaged by brushes 96 and 97 which are mounted on casing wall 31 and electrically connected through leads, not shown, to the contact or terminal strip 83. The rings 94 and 95 are also connected through electrically conductive screws 98 and 99 to the static contact assemblies 43 to which tap leads 48 and 49 are connected. More specifically, each screw 98 or 99 extends to and serves to mount a finger contactor 100 or 101 which forms one component of each static contact assembly 43. Each finger component 100 or 101 is in engagement with an arcuate contact strip 102 or 103 to which tap leads 48 and 49 are respectively connected.

As is shown clearly in Figure 9, the finger conta'ctors 100 or 101 are formed of appropriate lengths and each comprises an arcuate portion 104 formed integral with three U-shaped resilient fingers 106. The portion 104 is provided with suitable apertures 107 (Figure to receive the screws 98 or 99 which electrically associate the finger contactors with brush rings 94 or 95 and also serve to mount the contactors. The arcuate contact strips 102 and 103 (Figure 3) are suitably mounted on the base wall of annular groove 42 and are positioned to be engaged, respectively, by the fingers 106 of finger contactors 100 and 101. It will be evident that the provision of the static contact assemblies 43 facilitates the construction and assembly of the variable resistor structure. The arcuate strips 102 and 103 are first mounted in the bottom of annular groove 42 and suitably connected to tap leads 48 and 49 or other tap leads as may be desired. Spacer ring 40 is then assembled with brush rings 94 and 95, screws 98 and 99, and finger contactors 100 and 101. It is then merely necessary to press the spacer ring 40 into its recess in order to effect electrical connections be tween the appropriate rings 94 and 95 and tap leads 48 and 49.

In the operation of the variable resistor structure, let it be assumed that a mandrel 20 is provided which has an external surface of revolution formed by rotating a curved generatrix appropriate to the desired function. It is pointed out that the mandrel may be readily formed on a lathe or in any other suitable manner, there being no necessity for first forming an irregular surface and then wrapping the surface to form an internal resistance-supporting mandrel. Rotation of mandrel 20, through turning of shaft 21, will operate through gears 69-71 to rotate lead screw 59 for contact assembly 23 to axially shift the latter at a rate appropriate to maintain the resilient contact arms 26 and 27 in engagement with the resist ance element. The contact assembly 23 will then track along the resistance element 22 in the manner previously described in detail, and with the arms 26 and 27 flexing radially, and both flexing and pivoting axially, to insure that their bent-over ends 78 will remain in continuous engagement with the resistance element despite great irregularities in the contour of the mandrel.

If it is desired to use the variable resistor as a potentiometer, one or the other of the contact arms 26 and 27, which are electrically isolated from each other by the insulating base 24 of the contact assembly, is connected to terminal strip 83 and thus into an electric circuit. The brushes 91, 92, 96 and 97 for the various contactor rings 84, 85, 94 and 95 are thenconnected through terminal strip 83 to sources of voltage adapted to provide the desired voltage gradients. With the potentiometer connected as indicated, the connected contactor arm 26 or 27 will pick off a progressively increasing or decreasing voltage which varies in a non-linear manner in accordance with the diameter and slope of the mandrel 20 at the section where the resistance element 22 is contacted. Any desired linear or non-linear function may thus be generated by a variable resistor which is relatively inexpensive to manufacture and assemble. Referring to Figure 11, a schematic diagram is illustrated for the purpose of representing a typical potentiometer set-up. In this instance the contact arm 26 is illustrated as contacting the resistance element 22 which is associated at its ends with brushes 91 and 92 andat a point of inflection with tap lead 48. The diagram illustrates how the sliding of con tact arm 26 along resistance element 22 in a given direction will eifect both abrupt and gradual transitions in voltage level. v

When it is desired to employ the device as a rheostat of a highly novel type, the resistance element 22 is not connected in any circuit, the various brush and ring structures then being rendered unnecessary and are not utilized. Instead, one of the arms 26 is connected into a circuit the resistance of which it is desired to vary, and the other arm 27 is also electrically connected into the circuit. The amount of resistance inserted will then depend upon the distance between the resistance elementengaging ends 78 of contact arms 26 and 27 as measured along the resistance element. It follows that where the contact arms 26 and 27 are bridged across a section of resistance element at a wide point in mandrel 20, the amount of resistance inserted into the circuitwill be relatively great. Where, however, the arms are bridged'across a narrow point in the mandrel,- the amount of inserted resistance will be small. There are no upper or lower limits in the rheostat, and alternately increasing and decreasing resistance values may be progressively inserted into the circuit upon continuous unidirectional rotation of shaft 21. Furthermore, a resistance which neither increases nor decreases may be inserted by use of a mandrel 20 having a cylindrical section, so that the distance between the contactor ends remains constant while that section. is traversed. These features, or capabilities, are shown schematically in Figure 12.

Referring next to Figure 13, a potentiometer is fragmentarily illustrated which incorporates a different type of brush ring, for the resistance element ends, than that shown in Figure 1. Elements in Figure 13 which are similar to those illustrated in Figure 1 have been given the same reference numerals but followed by the letter a. In the illustrated modification, the brush ring 11 for each end of resistance element 22a is mounted between outer and inner flanges 112 at the peripheries of the ends of mandrel 20a. Each ring 111 is held in position by screws 113 which extend radially through the mandrel from an annular recess or groove 114 in the wall of the spacer ring recess. At its outer cylindrical surface, each ring is engaged by a brush 116 which is mounted on a bracket 117 on the adjacent casing end wall. Electrical connection between each ring 111 and resistance elements 22a is effected by a wire, not shown, which is inserted through a bore 118 in the inner of flanges 112.

Referring next to Figure 14, a second modified construction is illustrated, with similar components given the same reference numerals as in Figure 1 except that they are followed by the letter b. In this modified construction the mandrel 20b is formed with two independent helical grooves 121 and 122 only the latter of which is provided at its bottom with a groove adapted to receive the resistance element 22b. The other groove, numbered 121, is provided solely for the purpose of tracking the contactor arm 26b as shown in the drawing. The contact finger 77b of contact arm 26b makes electrical contact with resistance element 22b as previously discussed, while the outer and lateral guide portions b and 76b ride along mandrel shoulders 57b to track the contact finger 77b also as previously described. The only difference between the contact arm 26b and the arm 26, best shown in Figure 6, is in the provision of spacer arms 123 to span adjacent volutions of groove 121 in such a way that the shoulders 57b of such adjacent volutions, and remote from the section of groove 122 in which contact finger 77b is located, will serve as tracks for contact arm 26b.

While the particular apparatus herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations. are intended to the details of construction or design herein shown other than as defined in the appended claims.

I claim:

I. A variable resistor structure, comprising a mandrel journaled for rotation about a central axis, the exterior surface of said mandrel being a curved surface of revolution about said central axis, a resistance element wound helically on said exterior surface, guide means mounted generally parallel to said central axis and spaced radially outwardly from said mandrel, and an electrical contact element having a base portion movable along said guide means and a contact portion in engagement with said resistance element, said contact portion being adapted to track along and in engagement with said resistance element during rotation of said mandrel, and being adapted to move radially of said mandrel to compensate for variations in the diameter thereof, said contact portion including a pair of resilient arms adapted to engage said resistance element at substantially diametrically opposite portions of said mandrel.

2. A variable resistor structure, comprising a mandrel journaled for rotation about a central axis, the exterior surface of said mandrel being a curved surface of revolution about said central axis, a helical groove formed in said exterior mandrel surface and having its volutions separated by an exposed helical shoulder portion of said surface, a resistance element wound in said groove, a guide mounted generally parallel to said central axis and spaced radially outwardly from said mandrel, a contact base adapted to move along said guide, and a contact arm connected at one end to said base and shaped at the other end to both ride on said shoulder portion and electrically contact said resistance element.

3. The invention as claimed in claim 2, in which said other end of said contact arm is shaped with a guide adapted to ride on said shoulder portion, and with a separate resilient contact finger adapted to contact said resistance element.

4. The invention as claimed in claim 3, in which the main body of said contact arm is resilient in such a way as to effect pressing of said guide against said shoulder portion, said main body being stronger than said contact finger so that the pressure of said contact finger against said resistance element is determined solely by its own strength and resilience.

5. The invention as claimed in claim 2, in which said contact arm is adapted to flex both radially and axially of said mandrel.

6. The invention as claimed in claim 2, in which the connection between said contact arm and base is such that said arm may pivot about an axis transverse to said mandrel.

7. A rheostat, comprising a pair of mutually insulated contacts spaced a predetermined distance from each other, a non-linear resistance component adapted to be electrically contacted by both of said contacts, and means to relatively move said contacts and resistance component to cause said contacts to bridge varying amounts of resistance as the result of the non-linearity of said resistance component.

8. A potentiometer, comprising a casing, a first shaft journaled in said casing, an insulating mandrel mounted on said first shaft for rotation thereby, said mandrel having its exterior surface a curved surface of revolution and formed with a helical groove, a resistance wire wound in the bottom of said groove, brush and ring means at the ends of said mandrel to pass electricity through said resistance wire, a second shaft journaled in said casing parallel to said first shaft and spaced from said mandrel, said second shaft being formed with a helical thread having the same lead as said groove, gear means to associate said first and second shafts with a one-to-one ratio, a contact base mounted on said second shaft and adapted for feeding thereby, and a resilient contact arm mounted on said contact base and extending radially inwardly toward said mandrel, said contact arm being adapted to track on the shoulders of said mandrel groove and to electrically contact said resistance wire.

References Cited in the file of this patent UNITED STATES PATENTS 736,557 Shoemaker Aug. 18, 1903 2,114,330 Borden Apr. 19, 1938 2,405,890 Kunz Aug. 13, 1946 2,454,986 Beckman Nov. 30, 1948 2,615,065 Farnham Oct. 21, 1952 2,665,355 De Witt et al. Jan. 5, 1954 FOREIGN PATENTS 392,207 France May 15, 1933 

